Plants face various abiotic stresses in their natural environments that trigger the production of reactive oxygen species (ROS), leading to oxidative stress and potential cellular damage. This comprehensive review examines the interplay between plant antioxidant defense systems and ROS under abiotic stress conditions. We discuss the major enzymatic antioxidants, including superoxide dismutase, catalase, reductases, and peroxidases, as well as non-enzymatic antioxidants, such as ascorbic acid, glutathione, polyphenols, and flavonoids, which play crucial roles in ROS detoxification. This review elaborates on different types of ROS, their production sites within plant cells, and their dual role as both damaging oxidants and key signaling molecules. We discuss how various abiotic stresses—including heat, cold, drought, flooding, salinity, and heavy metal toxicity—induce oxidative stress and trigger specific antioxidant responses in plants. Additionally, the mechanisms of ROS generation under these abiotic stress conditions and the corresponding activation of enzymatic and non-enzymatic scavenging systems are discussed in detail. This review also discusses recent advances in understanding ROS signaling networks and their integration with other stress-response pathways. This knowledge provides valuable insights into plant stress-tolerance mechanisms and suggests potential strategies for developing stress-resistant crops by enhancing antioxidant defense systems. Moreover, the strategic ROS modulation through priming, exogenous antioxidants, nanoparticles, or genetic tools can enhance plant resilience. Integrating these methods with agronomic practices (e.g., irrigation management) offers a sustainable path to climate-smart agriculture. Our review reveals that ROS accumulation can be detrimental; however, the coordinated action of various antioxidant systems helps plants maintain redox homeostasis and adapt to environmental stress.

Chromium (Cr) is an element naturally occurring in rocky soils and volcanic dust. It has been classified as a carcinogen agent according to the International Agency for Research on Cancer. Therefore, this metal needs an accurate understanding and thorough investigation in soil–plant systems. Due to its high solubility, Cr (VI) is regarded as a hazardous ion, which contaminates groundwater and can be transferred through the food chain. Cr also negatively impacts the growth of plants by impairing their essential metabolic processes. The toxic effects of Cr are correlated with the generation of reactive oxygen species (ROS), which cause oxidative stress in plants. The current review summarizes the understanding of Cr toxicity in plants via discussing the possible mechanisms involved in its uptake, translocation and sub-cellular distribution, along with its interference with the other plant metabolic processes such as chlorophyll biosynthesis, photosynthesis and plant defensive system.

Citrus species are widely cultivated across the globe and frequently encounter drought stress during their growth and development phases. Previous research has indicated that citrus species synthesize flavonoids as a response mechanism to drought stress. This study aimed to comprehensively quantify and analyze the presence of 85 distinct flavonoids in the leaf and root tissues of lemon (drought susceptible) and sour orange (drought tolerant). In drought-stressed sour orange roots, flavonoids, such as isosakuranin, mangiferin, trilobatin, liquiritigenin, avicularin, silibinin, and glabridin, were more elevated than control sour orange roots and drought-stressed lemon roots. Additionally, hydroxysafflor yellow A, cynaroside, tiliroside, and apigenin 7-glucoside were increased in drought-stressed sour orange leaves compared to drought-stressed lemon leaves. Under drought stress, flavonoids such as (-)-epigallocatechin, silibinin, benzylideneacetophenone, trilobatin, isorhamnetin, 3,7,4′-trihydroxyflavone, and liquiritigenin were significantly increased, by 3.01-, 3.01-, 2.59-, 2.43-, 2.07-, 2.05-, and 2.01-fold, in sour orange roots compared to control sour orange roots. Moreover, the total flavonoid content and antioxidant capacity were significantly increased in drought-stressed sour orange leaves and root tissues compared to drought-stressed lemon leaves and root tissues. The expression levels of genes involved in flavonoid biosynthesis were highly expressed in sour orange leaves and roots, compared to lemon leaves and root tissues, post-drought stress. These findings indicate that lemons fail to synthesize protective flavonoids under drought conditions, whereas sour orange leaves and root tissues enhance flavonoid synthesis, with higher antioxidant activities to mitigate the adverse effects of reactive oxygen species generated during drought stress.

Nowadays, drought is one of the major abiotic factors which negatively affects growth and development of several fruit tree species, including Chinese hickory plants (Carya cathayensis Sarg.). The present investigation was conducted to study the possible positive effects of melatonin in drought resistance of C. cathayensis plants along with associated mechanisms. It was observed that melatonin pre-treatment applied before limited water availability significantly contrasted drought-promoted negative effects in terms of plant growth and physiological responses. Significant improvement was observed in key biological parameters like relative water content, net photosynthetic rate, stomatal conductance, transpiration rate, maximum photosynthetic efficiency of photosystem II (PSII), and PSII electron transport rate. Antioxidant apparatus was also stimulated by melatonin and enhanced activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) were noticed along with higher accumulation of proline. Gene expression studies herein revealed that melatonin promoted the up-regulation of the expression of SOD (70.7%), CAT (32.7%), and APX (66.5%) genes. As a consequence, accumulation of malondialdehyde by-products and leaf symptoms were reduced in melatonin-treated plants. All these observations offer the clear evidence that pre-treatment with melatonin ameliorate the performance of Chinese hickory plants against drought stress.